Use of ACR Method to Estimate Closure and Residual Stress Free Small Crack Growth Data

Abstract

Abstract Life predictions for small cracks emanating from material inhomogeneities or small flaws caused by manufacturing or corrosion damage are becoming increasingly important for several reasons, including: concerns over widespread fatigue damage; a shift in philosophy for fatigue initiation design from stress-life (S-N) to the equivalent initial flaw size (EIFS) approach, and the greater usage of unitized structure. For these reasons, development of accurate fatigue crack growth rate (FCGR) data in the near threshold region is needed, which is not influenced by closure or residual stress. Threshold data ARE often generated from long crack specimens by load shedding as outlined in ASTM E 647. However, this data can be non-conservative and variable due to load history effects caused by crack closure. Generating closure-free short crack FCGR data requires much more sophisticated instrumentation and testing techniques than long crack testing, making it prohibitively expensive to perform on a routine basis. Small crack tests also sample only a small volume of material and typically exhibit large scatter, so multiple tests must be performed in order to determine the typical or average behavior of the material. As a result of these disadvantages, use of long crack tests to estimate short crack behavior is the preferred approach. The objective of this study was to evaluate the adjusted compliance ratio (ACR) method for obtaining residual stress and closure-free data from long crack tests and to compare the ACR method against the current ASTM opening load method. The ACR method was developed by Donald for estimating the closure-free ΔKeff FCGR curve from standard long crack tests. To evaluate the method's ability to estimate closure-free data, standard ASTM long crack tests and short corner crack tests were conducted on 7075-T651 and 2324-T39 alloys. The ΔKeff FCGR data resulting from the application of the ACR method to the ASTM long crack test results were then compared with the closure-free short corner crack data. Additionally, to assess the method's ability to account for residual stresses, long crack tests using ASTM standard middle crack and compact tension specimens, prepared from alloy 7055-T74511, were conducted, and the resulting ΔKapplied and ΔKeff FCGR data were compared.